There are many different types of door locks being employed to prevent and deter breaking and entering into an enclosure such as a residence. Certain locks employed are sometimes used as security locks for a door and permit limited opening of the door for observation purposes. Such locks would generally employ chains with some play to permit limited opening of the door. The chain is mounted at one end to the door frame and at the other end to the door. Unfastening of the chain is accomplished by means of a mechanical structure such as a sliding bolt and keyway arrangement. It has been found on occasion that this type of locking device can be relatively easily opened by extending a tool or hand through the limited opening in the door and unfastening the lock from the exterior of the door or by cutting the chain.
It would therefore be of value to have a lock of the type which can be used as a secondary lock to permit limited opening of the door for observation purposes while employing a locking arrangement which makes unfastening or breaking of the lock extremely difficult.